Please answer this math question?

Recycles CDs, incorporated, offers a choice of 5 used CDs for $25, with each additional CD costing $4. Write a cost function for purchasing 5 or more CDs. What is the cost of buying 11 CDs. Let x represent the number of CDs.
I need an expression using x as a variable
15 d
Also sorry for a stupid question but it’s the only question on my sisters homework that I cannot figure out!!
Please answer this math question?
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